To pre-empt the theft, Brown enters a battle of wits to ensure that the item, the Blue Cross, is not stolen, and in the process discovers there is a deep connection between himself and Flambeau, linked with the Great War.
Violet Fernsley (Emma Stansfield) – convicted of killing her pig-farmer husband, whose body has never been found – is given a temporary reprieve from the gallows when she claims she is pregnant.
Professor Hilary Ambrose (James Laurenson) sends an ancient prayer book containing a clue to its whereabouts to his old friend Father Brown, before he goes missing and his home is ransacked.
Sid is arrested when he and Father Brown investigate mysterious activities at a local stately home, and Lady Felicia is blackmailed by MI5 spymaster Daniel Whittaker looking for a Soviet agent who has infiltrated a secret intelligence base there.
Father Brown – with the aid of Mrs. McCarthy, Lady Felicia, and Sid – rallies round to try to prove his innocence and protect him from a deadly organisation with members highly placed in the community.
Following her latest scandal, Bunty's father insists she stay in Kembleford, and Lady Felicia needs her to look after her home as she is leaving with her husband, Monty, who has been appointed Governor of Northern Rhodesia.
Sid Carter is released from prison after a year following his conviction of assault (of which he was innocent) on Judith Miles, then a prostitute now working in a solicitor’s office.
Two items catch Father Brown's eye in the newspaper: Flambeau's death in Italy and the Iron Crown of Lombardy believed to contain a nail from the True Cross on display at Gloucester cathedral.
Opera diva Bianca Norman offers Lady Felicia and Mrs. McCarthy passage in her personal Pullman train carriage with her adopted children, latest husband, and staff.
Nicholai Solovey – who previously had offered to buy her necklace, which he claims belonged to his family – steals it and places it in an impregnable safe in his hotel room.
Mrs. McCarthy arranges a coach to take the choristers – including Sergeant Goodfellow and Lady Felicia, Inspector Mallory, whose daughter is singing in a junior competition, and Father Brown.
Reginald Brody is found dead at the bottom of a tower owned by Alistair Hemsley MP, an apparent suicide by jumping from the sealed roof.
The next day Valentine and Hemsley's brother William, who had purchased a red scarf the morning before Brody's body was found, go back to the top of the tower.
A stolen car and a man known to Sid's prison contacts, who Mallory wronged ten years ago, are potential keys to the Inspector's recent behavior.
Gerald Firth (Kalon – see Series 1, Episode 5 "The Eye of Apollo") beseeches Father Brown to speak on his behalf at the mental health facility where he has been confined.
During the visit, owner Marjorie Chummy – who'd been the target of some nasty pranks – is strangled with an apron belonging to camp maid and general dogsbody Mavis Jug.
Inspector Mallory arrests her while Father Brown and Sergeant Goodfellow look at the other camp staff – Jock McCudgeon, Griff Grimshaw, and Sandy Beauchamp – who were kept under a tight rein by Mrs Chummy but who were on stage when her death was heard on the loudspeaker.
Father Brown discovers that a child, Billy Fairfield, had died in the camp swimming pool several years earlier when Mavis Jug, then an "Orangecoat" entertainer and in charge of first aid, was away from the area and Mrs Chummy did not try to save him.
When Joe Telford and Jennifer Mossop, two council members opposed to the development, are murdered, Father Brown notices their figurines in the model were moved to the locations where they were killed before their deaths.
Brenda Palmer, whom Father Brown met at a borstal (youth detention center) for girls, arrives at his door asking for help after being accused of theft by Jasper Granford, the owner of the gentlemen's club where she worked.
Flambeau believes he is being framed for thefts and murders across Europe when valuables he stole 25 years earlier are re-stolen and his handkerchief calling card left at the scene.
Having received an anonymous tip about Flambeau's presence, Inspector Sullivan arrives at the auction house just as a security guard is murdered and the bracelet stolen.
All episodes for this series became available on the BBC iPlayer from the 5 January 2024 The 1955 Kembleston Olimpicks between the villages of Kembleford and Hambleston bring murderous tensions between the teams with accusations of cheating and contestants unable to compete.
With Mrs Devine's and Sergeant Goodfellow's help, Father Brown frees Sister Boniface from jail without Inspector Sullivan's knowledge, allowing her to prove her innocence.
He discovers that his father lured him there to help recover a gold crown belonging to the Vatican, which Hawksworth had stolen from a museum for crime baron Drake Underwood.
Lady Felicia asks Father Brown to investigate the death of her goddaughter, Sophie Blackthorn, believed to be the fourth victim of The Bride Killer, William Harrow, who is awaiting hanging.
Father Brown, Lady Felicia, Mrs Devine, and Brenda go to the Blackthorn Institute, a college for criminal studies set up in memory of Sophie and run by Ralph and forensic pathologist Professor Pritchard.
Attending the memorial are Eddie Divine wearing his father's medals, widow Rachael Waterson, Private John Carlton, conscientious objector Oscar Treadwell, and overbearing and vindictive Captain Fred Howton.
When an attempt on Thorncastle's life fails, Father Brown investigates and discovers motives for the other two couples and the TV producer; one with, for the time, a secret that could have him jailed, another with a past criminal record, and another mourning the death of a brother.
Father Brown, Mrs Divine, and Brenda having received a letter from a resident, Adelaide Jenks, they knew at Thorp's Almhouses for women requesting spiritual advice.